Process and apparatus for the direct and continuous modification of polymer melts

ABSTRACT

A process and an apparatus for the direct, continuous and flexible modification of polymer melts by branching off a branch melt stream from the main melt stream to be modified, dispersing the initially introduced additive in the melt in a specially designed twin-screw extruder, and subsequent recycling of the additive melt concentrate to the melt stream to be modified.

This application is a continuation of U.S. application No. 121,682,filed Sep. 14, 1993, now abandoned, which is a division of applicationNo. 08/780,144, filed Oct. 27, 1991, now U.S. Pat. No. 5,396,702, which,in turn, claims the priority of German Applications 40 33 319.9, filedOct. 19, 1990 and 40 39 857.9, filed Dec. 13, 1990.

The invention relates to a process for the direct, continuousincorporation of additives into polymer melts in a side stream and isdistinguished by a very high degree of flexibility with respect toproduct change-over.

BACKGROUND OF THE INVENTION

For various reasons, polymer melts have to be modified with additives.For example, such melts are substantially transparent because thehomogeneous structure of the synthetic polymers does not permitrefraction or reflection of light. Therefore, for applications,pigments, such as TiO₂, ZnS, or carbon black, are added to polymermelts. It is desirable to rid the melt before spinning of theunattractive, greasy luster which has a disturbing esthetic effect,especially when the fibers are to be used for clothing. The eliminationof the luster takes place by adding a relatively small amount of awhitening pigment, usually titanium dioxide. The modification of polymermelts by the use of additives for other purposes is also known; e.g. forproducing antipilling fibers or for the optical brightening of polymerswith e.g. a yellow tinge.

With the increasing output of continuous polycondensation plants, onlyproduction lines with a high degree of flexibility are economical.Often, the parallel production of different types of products, such asfiber and granules, must be possible at the same time on a singleproduction line. In such cases, the change-over to different products,such as for example, delustered, lustered, or pigmented, should beeffected within the shortest time and, as far as possible, with no loss.These requirements are met only inadequately by the methods availableheretofore.

It is well known that melts can be modified with the help of masterbatchproducts. This principle is described in German Offenlegungsschrift 1604 368. A melt from masterbatch granules and containing a highpercentage of additive is introduced into the polymer melt in a definedratio. Although this process offers high flexibility with respect tochange-overs of finish, it has substantial disadvantages. First, themasterbatch granules must be dried before melting; second, duringremelting they undergo additional thermal and mechanical loading; andthird, there are additional handling costs. Another disadvantage is therisk of external soiling of the granules. In the case of in-housemasterbatch production, there is the added drawback that polymergranules have to be taken from the production process. If themasterbatch granules are obtained elsewhere, there are likely to bedifferences between the polymers used which have an adverse effect onthe properties of the final product.

A variant of the above process is described in Chemical EngineeringProgress 78 (1982) 1, pages 62-64. Instead of introducing the additiveinto the polymer melt downstream, both the polymer granules and thepreferably reactive additives are introduced into the first extruderzone. The disadvantages of this process are, on the one hand, the riskof compacting of the additive between the granules and, on the otherhand, the high shearing stresses which are necessary for homogenizationand dispersion. The latter result in a large, uncontrollable reductionin the viscosity of the polymer.

A further state of the art process is described in Chemiefasern undTextilindustrie 1 (1986), pages 24 to 29. A partial stream is branchedoff from the main stream emerging from the polycondensation finisher andthe additive is incorporated into the melt downstream. Thisadditive-charged melt is recycled into the main melt stream. Thisprocess also has a high degree of flexibility in additive change-overs,but also several major disadvantages. One is that, in spite ofevacuation, the volatile constituents contained in the polymer melt,such as glycol, water, or oligomers, due to vaporization thereof, escapethrough the filling funnel during the feeding-in of the additives. Thisresults in reduced free-flowability of the additive and increases lumpformation. Additive agglomerates can no longer be incorporatedhomogeneously into the melt and, during the spinning process, they causeclogging of the filters as well as weak points in the filaments. Inaddition, continuous, uniform metering of the additive is impaired bythe thermal convection resulting from the hot melt and barrel, whichkeeps the additive particles, usually fed in free falls, in suspensionat the feeding point. A further disadvantage is that the extruderopening for the feeding of the additive must be kept small in order toreduce the risk of melt escaping at this point. Consequently, the amountof additive is also limited.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a process and anapparatus for the direct, continuous modification of polymer meltswithout the above disadvantages.

In the process according to the invention, polymer melts, preferablypolyester, polyamide, or polyolefin, most preferably polyester, aremodified with additives. Such additives may be soluble or insoluble,mineral or organic pigments, such as ZnS, SiO₂, and TiO₂, preferablyTiO₂, or other melt-modifying additives improving the properties of theproduct, such as optical brighteners and light and heat stabilizers,preferably optical brightening agents.

In the process according to the invention, the additive, which may alsobe mixture, is initially introduced alone into the first extruder zoneand the melt stream is fed in in the second zone. Subsequently, theadditive is wetted with the polymer melt, the mixture is passed througha degassing zone and, furthermore, is homogeneously dispersed in knownmanner and conveyed to the extruder outlet. The wetting of the additive,initially introduced by itself, with the polymer melt in the closedextruder filled to as great an extent as possible has the advantage thatno volatile melt constituents can leave rearwards and adversely affectthe feeding in of the additive. This is assisted by the evacuation,which is arranged downstream and carries away even volatileconstituents, such as moisture, from the additive. In order to avoid acompacting or briquetting of the unwetted additive until melt entry, theheight of the flight land of the screws in this zone must be reduced, ascompared to those of the screws in the other zones. According toexperience, reduction of the screw diameters of 0.2 to 4 mm, preferablyof 0.5 to 2 mm, is adequate for this. 0.5 to 1 mm is particularlypreferred for delustering agents such as titanium dioxide and polyesteror polyamide as the polymer. The dispersing and conveying of theadditive-polymer mixture takes place in the conventional way by means ofsuitable screw elements in the remaining extruder zones.

In the process according to the invention, the wetting by polymer meltis possible with additives which satisfy a special test method--the socalled pressure test--in which the mass throughput per minute of a meltsimulating substance through a filter of defined mesh width isdetermined at the beginning of the test and after 30 minutes. Forpolyterephthalate melts and polyamide 6 melts, use on an industrialscale is ensured for (1) extrusion and injection-molding with a residualquantity of 90%, (2) fiber and filament spinning with a residualquantity of 60%, and (3) for POY and FOY yarns with a residual quantityof 40%.

Pressure test: of a melt simulating substance at the beginning of thetest

In a model substance (e.g. polyethylene glycol) the same viscosity asthe polymer melt to be modified at the corresponding temperature, theadditive is dispersed, heated to the measuring temperature and passedthrough a filter of 11 to 13 μm nominal width. The amounts of materialsflowing through the filter are then determined, m₁ in the time intervalt₁ which is the sixth minute of flow, and m₂ in the time interval t₂which is the 36th minute of flow; the drop (D) in material throughput(in percent) is

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, constituting a part hereof and in whichlike reference characters indicate like parts,

FIG. 1 is a schematic flow diagram of the inventive process; and

FIG. 2 is a schematic view of an extruder in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring specifically to FIG. 1, main melt stream 1 carries all of thepolymer to be processed. At point 20, it divides into unmodified stream17 and branch stream 2 which is to receive the additives. Side stream 3takes a portion of the flow in branch stream 2, and feed pump 4 deliversit to melt entry zone 8 of extruder 6. The additive is introduced byadditive feed 5 into metering zone 7. The additive is carried byextruder 6 from zone 7 to melt entry zone 8 and thence, along with thepolymer melt, to wetting zone 9. As the mixture continues its flow, itpasses through degassing zone 10 into dispersing zone 11. Degassingdevice 12 removes volatiles from zone 10.

Thereafter, the stream, containing the desired additives, is returned tobranch stream 2 by return pump 13 via modified side stream 14. Themixture enters static mixer 15 where blending with branch stream 2 iscompleted and, thereafter, flows through spinning point 16 to be madeinto fiber.

Simultaneously, unmodified stream 17 is directed to granulator 18 forformation into raw granules. In this manner, a single production line iscapable of producing two types of polymers simultaneously oralternatively. There is no "down time" of the line if it is desired toswitch from one to the other.

With the process and apparatus of the invention, polymer melts can bemodified by the introduction of 0.01 to 16% by weight, preferably 0.04to 0.6% by weight, particularly preferably 0.4% by weight, of additivebased on the melt polymer.

Referring more particularly to FIG. 2, apparatus in accordance with thepresent invention is schematically shown. The additive is metered intometering zone 21 at 30 and the polymer melt enters melt entry zone 22 at31. The additive is transferred by the screw of extruder 19 frommetering zone 21 to melt entry zone 22. The mixture of polymer melt andadditive then moves into degassing zone 23 where the volatiles areremoved as indicated by arrow 32. Once this is accomplished, the mixturethen flows through dispersing zones 24 through 28 and exists at screwnose 29.

A suitable extruder is type ZE 40 A (produced by Messrs. Berstoff)having heatable and coolable zones. This device comprises a co-rotatingtwin screw and is specially designed as described herein. It has beenfound particularly suitable if the diameters of the screw elements arereduced by 0.2 to 4.0 mm, preferably by 0.5 to 2 mm, and most preferablyby 0.5 to 1 mm. Melt entry zone 8 has known conveying elements andwetting zone 9 is provided with kneading and retarding elements.Degassing zone 10 also has known conveying elements and dispersing zone11 is equipped with both conveying and kneading elements. Metering zone7 can be cooled if temperature-sensitive additives are used.

Additive feeding device 5 is preferably a dropping pipe. Of course,other additive feed means can be substituted if desired or convenient.

The process and apparatus according to the invention provide a highdegree of flexibility. Additive change-overs can be performedimmediately and with virtually no loss of time, without expensivecleaning operations and without the formation of inferior productfinishes. Additive carrier polymers are not necessary and, therefore,there are no disadvantageous effects on the properties of the finalproduct. There is no need for complex interventions in the productionprocess, such as the production of in-house masterbatches, no escape ofpolymer melt and/or volatile fractions during introduction of theadditives; agglomerate-free modifying of the melt and consequentlyprolonged filter service lives. Moreover, the invention achievesconsiderably lower screw speeds than are necessary in the prior art, nopredrying of the additive is required, and only minimal, constantviscosity reduction of the polymer melt occurs. Due to the presentinvention, it is possible to dimension the extruder additive feedopening as large as the entire screw cross-section. There is nolimitation of the quantity of additive fed in or of the melt throughputdepending on the extruder size; continuous, uniform metering-in of theadditive into the first extruder zone without thermal conductionproblems is obtained; the possibility that the first extruder zone canbe cooled if temperature-sensitive additives are used; and the use ofprocessing aids as in the prior art is not necessary.

The following examples illustrate preferred embodiments of the inventionwithout restricting it.

EXAMPLE 1

0.72 kg/h of titanium dioxide (Messrs. Sachtleben, type LOCR-S-U) ismetered into the first zone of a twin-screw extruder equipped with thescrew configuration according to FIG. 2 and cooled to 50° C. It isconveyed by the screws having diameters reduced by 0.5 mm into thesecond extruder zone. In parallel, a partial stream to be modified, of120 kg/h, is divided from the polyethylene terephthalate melt mainstream and the side stream of 9 kg/h is in turn branched off the partialstream and directed onto the titanium dioxide which has been conveyedinto the second extruder zone.

The further incorporation of the titanium dioxide into the polymer melttakes place with the following machine setting data, the quoted barreltemperatures being achieved by cooling or heating. The resultingtitanium dioxide-delustered side melt concentrate contains 8% by weightof TiO₂, which corresponds to a final pigment content of 0.6% by weightof TiO₂ in the modified melt stream.

    ______________________________________                                        Temperature of the polyethylene terephthalate melt                                                        290° C.                                    side stream on entry into the second extruder zone:                           Speed of extruder:          250 rpm                                           Power consumption of extruder:                                                                             8.8 A                                            Applied voltage:            385 V                                             Extruder counterpressure:    40 Bar                                           Temperature of the modified poly-                                                                         270° C.                                    ethylene terephthalate side melt                                              stream after leaving the extruder:                                            Barrel temperatures:                                                          zone 1                       50° C.                                    zone 2                      225° C.                                    zone 3                      265° C.                                    zone 4                      265° C.                                    zone 5                      270° C.                                    zone 6                      270° C.                                    zone 7                      270° C.                                    zone 8                      270° C.                                    nose                        265° C.                                    ______________________________________                                    

EXAMPLE 2

0.32 kg/h of titanium dioxide (Messrs. Kronos, type AVF 9009) is meteredinto the first zone of the twin-screw extruder of FIG. 2 and cooled to50° C. It is then conveyed by the screws in the first extruder zone,having diameters reduced by 1 mm, into the second extruder zone. Inparallel, a partial stream to be modified of 53 kg/h, is divided from apolyamide 6 melt main stream and a side stream of 8 kg/h is in turnbranched off the partial stream and directed onto the titanium dioxidewhich has been conveyed into the second extruder zone.

Further incorporation of the titanium dioxide into the polyamide melttakes place with the following machine settings, the quoted barreltemperature being achieved by cooling or heating. The resulting titaniumdioxide-modified polyamide side melt concentrate contains 4% by weightof TiO₂, which corresponds to a final pigment content of 0.6% by weightof TiO₂ in the modified part melt stream.

    ______________________________________                                        Temperatures of the polyamide 6 melt side stream on                                                       280° C.                                    entry into the second extruder zone:                                          Speed of extruder:          150 rpm                                           Power of consumption of extruder:                                                                          9 A                                              Applied voltage:            230 V                                             Extruder counterpressure:    40 bar                                           Temperature of the modified polyamide                                                                     277° C.                                    6 side melt stream after leaving the                                          extruder:                                                                     Barrel temperatures:                                                          zone 1                       50° C.                                    zone 2                      225° C.                                    zone 3                      263° C.                                    zone 4                      265° C.                                    zone 5                      270° C.                                    zone 6                      270° C.                                    zone 7                      270° C.                                    zone 8                      265° C.                                    nose                        265° C.                                    ______________________________________                                    

EXAMPLE 3

The process according to Example 1 was carried out except that theextruder settings were as follows:

    ______________________________________                                        Screw diameter reductions in the first                                                                  1 mm                                                extruder zone:                                                                Polyethylene terephthalate melt                                                                       3000 kg/h                                             main stream:                                                                  Polyethylene terephthalate melt                                                                        75 kg/h                                              side stream:                                                                  Titanium dioxide (Messrs.                                                                              12 kg/h                                              Sachtleben, type LOCR-S-U):                                                   TiO.sub.2, content of the modified side                                                                16% by weight                                        stream:                                                                       Final TiO.sub.2 content of the modified                                                               0.4% by weight                                        part melt stream:                                                             Temperature of the polyethylene                                                                        292° C.                                       terephthalate melt side stream                                                on entry into the second extruder                                             zone:                                                                         Speed of extruder:       250 rpm                                              Power consumption of extruder:                                                                         21 A                                                 Applied voltage:         240 V                                                Extruder counterpressure:                                                                              20 bar                                               Temperature of the modified                                                                            281° C.                                       polyethylene terephthalate side melt                                          stream after leaving the extruder:                                            Barrel Temperatures:                                                          zone 1                   30° C.                                        zone 2                   240° C.                                       zone 3                   260° C.                                       zone 4                   260° C.                                       zone 5                   260° C.                                       zone 6                   260° C.                                       zone 7                   260° C.                                       zone 8                   260° C.                                       nose                     260° C.                                       ______________________________________                                    

EXAMPLE 4

The process of Example 1 was followed except that the polyethyleneterephthalate side stream was modified with an optical brightener(Messrs. Eastman Kodak, type Optical brightener OPT-1) with thefollowing settings in the extruder.

    ______________________________________                                        Screw diameter reductions in the                                                                       0.5 mm                                               filest extruder zone:                                                         Polyethylene terephthalate melt                                                                       120 kg/h                                              main stream:                                                                  Polyethylene terephthalate melt                                                                        9 kg/h                                               side stream:                                                                  Optical brightener:      36 g/h                                               Content of optical brightener in the                                                                  0.4% by weight                                        modified side melt stream:                                                    Final content of optical brightener                                                                   0.3% by weight                                        in the modified part melt stream:                                             Temperature of the polyethylene                                                                       290° C.                                        terephthalate melt side stream                                                on entry into the second extruder                                             zone:                                                                         Speed of extruder:      250 rpm                                               Power consumption of extruder:                                                                         8.8 A                                                Applied voltage         385 V                                                 Extruder counterpressure:                                                                              40 bar                                               Temperature of the polyethylene                                                                       270° C.                                        terephthalate side melt stream,                                               modified with the optical                                                     brightener after leaving the extruder:                                        Barrel temperatures:                                                          zone 1                   50° C.                                        zone 2                  225° C.                                        zone 3                  265° C.                                        zone 4                  265° C.                                        zone 5                  270° C.                                        zone 6                  270° C.                                        zone 7                  270° C.                                        zone 8                  270° C.                                        nose                    265° C.                                        ______________________________________                                    

While only a limited number of specific embodiments have been expresslydisclosed, it is, nonethless, to be broadly construed and not to belimited except by the character of the claims appended hereto.

What we claim is:
 1. An apparatus for introduction of at least oneadditive into a polymer melt, said apparatus comprisingan extruderhaving a plurality of sequential zones comprising, in downstreamsequence, a metering zone, a melt entry zone, a degassing zone, and adispersing zone, at least one rotatable screw within said extruder andextending longitudinally into or through each of said sequential zones,said screw having smaller diameter flights within said metering zonethan in any of said entry zone, said wetting zone, said degassing zone,and said dispersing zone; a feed pump in fluid connection with saidentry zone and a source of said polymer melt, said feed pump adapted tointroduce said polymer melt into said entry zone, an additive feeder influid connection with said metering zone and a source of said additive,said additive feeder adapted to introduce said additive into saidmetering zone when said metering zone is free of polymer melt; saiddegassing zone being under lower pressure than said metering zone, saidentry zone, said wetting zone, and said dispersing zone, a degassingdevice in fluid connection with said degassing zone and adapted toremove gases from said melt; a return feeder in fluid connection withsaid dispersing zone and a side stream, said side stream bypassing saidextruder, said return feeder adapted to discharge said melt from saiddispersing zone into said side stream at a point, a mixing element insaid side stream at or downstream of said point.
 2. An apparatus forintroduction of at least one additive into a polymer melt, saidapparatus comprisingan extruder having a plurality of sequential zonescomprising, in downstream sequence, a metering zone, a melt entry zone,a degassing zone, and a dispersing zone, at least one rotatable screwwithin said extruder and extending longitudinally into or through eachof said sequential zones, a gap between an outer diameter of screwflights and an inner wall of said extruder is larger in said meteringzone than in any of said entry zone, said wetting zone, said degassingzone, and said dispersing zone; a feed pump in fluid connection withsaid entry zone and a source of said polymer melt, said feed pumpadapted to introduce said polymer melt into said entry zone, an additivefeeder in fluid connection with said metering zone and a source of saidadditive, said additive feeder adapted to introduce said additive intosaid metering zone when said metering zone is free of polymer melt; saiddegassing zone being under lower pressure than said metering zone, saidentry zone, said wetting zone, and said dispersing zone, a degassingdevice in fluid connection with said degassing zone and adapted toremove gases from said melt; a return feeder in fluid connection withsaid dispersing zone and a side stream, said side stream bypassing saidextruder, said return feeder adapted to discharge said melt from saiddispersing zone into said side stream at a point, a mixing element insaid side stream at or downstream of said point.
 3. The apparatus ofclaim 2 wherein said gap is 0.2 to 4.0 mm larger than in said entryzone, said wetting zone, said degassing zone, and said dispersing zone.4. The apparatus of claim 2 wherein said gap is 0.5 to 2.0 mm largerthan in said entry zone, said wetting zone, said degassing zone, andsaid dispersing zone.
 5. The apparatus of claim 3 wherein said gap is0.5 to 1.0 mm larger than in said entry zone, said wetting zone, saiddegassing zone, and said dispersing zone.
 6. The apparatus of claim 1wherein there is a wetting zone between said melt entry zone and saiddegassing zone.
 7. The apparatus of claim 1 wherein said metering zoneis cooled by a cooling device.
 8. The apparatus of claim 1 wherein saidadditive feeder is a dropping pipe.
 9. The apparatus of claim 1 whereinsaid extruder has twin screws.
 10. The apparatus of claim 6 wherein saidwetting zone has retarding and kneading elements.
 11. The apparatus ofclaim 1 wherein said dispersing zone has conveying elements and kneadingelements.
 12. The apparatus of claim 11 wherein said conveying elementsalternate with said kneading elements.
 13. The apparatus of claim 1wherein said mixing element is static.
 14. The apparatus of claim 9wherein said twin screws are co-rotating.
 15. The apparatus of claim 1wherein said smaller diameter is 0.2 to 4.0 mm smaller than in saidentry zone, said wetting zone, said degassing zone, and said dispersingzone.
 16. The apparatus of claim 15 wherein said smaller diameter is 0.5to 2.0 mm smaller than in said entry zone, said wetting zone, saiddegassing zone, and said dispersing zone.
 17. The apparatus of claim 16wherein said smaller diameter is 0.5 to 1.0 mm smaller than in saidentry zone, said wetting zone, said degassing zone, and said dispersingzone.
 18. The apparatus of claim 1 wherein said degassing device is asource of vacuum.